In its more general aspect, the present invention relates to a high transverse-curvature tire for two-wheeled vehicles.
More particularly, the invention relates to a tire preferably, though not exclusively, usable in the so-called high-performance xe2x80x9ctouringxe2x80x9d motor-bikes, of the type comprising:
a carcass structure including at least one ply provided with reinforcing cords essentially parallel to one another and oriented along directions inclined with respect to the equatorial plane of the tire;
a belt structure coaxially extending around the carcass structure and including a radially inner layer comprising at least one strip made of an elastomeric material including a plurality of reinforcing cords essentially parallel to one another and oriented along directions inclined with respect to the equatorial plane of the tire;
a tread, coaxially extending around the belt structure.
In the field of production of tires for two-wheeled vehicles, and in particular, those intended for equipping the so-called xe2x80x9ctouringxe2x80x9d motor-bikes generally having a swept volume exceeding 1000 cm3, a high weight and a high torque, the need is increasingly felt of providing ever higher performances in terms of stability of the vehicle at high speed, kilometric yield, wear uniformity and regularity, road behavior both in straight stretch and in curve, and low weight.
In order to fulfill this need, tires for two-wheeled vehicles have been manufactured for a long time with a carcass structure comprising a couple of plies of rubberized fabric reinforced with cords symmetrically inclined with respect to the equatorial plane of the tirexe2x80x94which carcass structure is also known as cross-plies carcassxe2x80x94and possibly an intermediate structure (breaker), also realized with couples of strips of rubberized fabric provided with cords arranged at an angle with respect to the tire equatorial plane.
Even though this tire structure could ensure an extremely regular curve behavior of the motor-bike, the use of this type of tires involved problems of comfort, stability of the vehicle at high speed, and especially of irregular wear of the tread, often associated with removal of bulk portions of rubber according to a phenomenon referred to in the art by the term: xe2x80x9cchunkingxe2x80x9d.
To try to prevent these drawbacks, it has been suggested to use a tire having a radial carcass and provided with a belt structure comprising at least a couple of strips of rubberized fabric with cords arranged at an angle with respect to the equatorial plane of the tire or, alternatively, a coil of cords, preferably made of metal, oriented in a circumferential direction and known in the art by the term of: zero-degree cords.
Even though this type of tire has improved the situation as concerns comfort and stability of the vehicle at high speed along a straight-away with a substantial elimination of chunking phenomenaxe2x80x94their low stiffness in the transverse direction markedly penalize their curve behavior.
Actually, the motor-bikes equipped with this type of tire are particularly subject to an undesired xe2x80x9cfloatationxe2x80x9d effect while running along a curve, otherwise known by the term of xe2x80x9cpudding effectxe2x80x9d, which causes the motor-bike to undergo a swaying phenomenon that would not be easily damped and that, in some cases, may be even self-amplifying. In extreme conditions, this phenomenon may even lead drivers to lose control of the vehicle, with obviously deleterious consequences.
According to the present invention, the Applicant has now understood that the problem of achieving adequate performances in terms of vehicle stability at high speeds, kilometric yield, wear regularity and uniformity, road behavior, both in straight stretches and in curves, and low weight can be solved by combining a carcass structure having at least one ply provided with reinforcing cords inclined with respect to the equatorial plane of the tire, with a belt structure including, in combination, at least one strip made of an elastomeric material incorporating a plurality of reinforcing cords inclined with respect to the equatorial plane of the tire and a radially outer layer comprising a plurality of zero-degree cords wound on the strip according to a preferred winding direction.
In a first aspect, the present invention therefore provides a tire of the aforementioned type, which is characterized in that the belt structure further comprises a radially outer layer including a plurality of circumferential coils, axially arranged side by side, of at least one circumferentially inextensible cord wound at a substantially null angle with respect to the equatorial plane of the tire according to a winding direction adapted to compression-stress elastomeric material portions interposed between consecutive reinforcing cords incorporated in said at least one strip of the radially inner layer.
More specifically, the Applicant has found that the best results are obtained when the winding direction of said circumferentially inextensible cord, at the starting end of the winding, forms with the reinforcing cords of the radially inner belt layer an acute angle, measured moving away from the equatorial plane of the tire, having a value not lower than 25xc2x0.
According to the invention, the Applicant has in particular found that such tire structure achieves at the same time both the desired properties of comfort, wear regularity and uniformity and stability of the vehicle at high speeds along a straight-away and the desired properties of road holding, curve stability and low weight.
Thanks to the structural features of the carcass structure, in fact, the tire of the invention can develop high camber thrusts quite sufficient to balance the centrifugal thrust affecting the motor-bike, with a very homogeneous curve behavior.
Thanks to this feature, it was observed that a motor-bike equipped with a couple of tires having such a structure affords neutral driving behavior, so that the driver can travel the curvilinear trajectory by simply inclining the vehicle and without correcting its trim, in particular without correcting the steering angle of the handlebar.
According to the invention, furthermore, it was also unexpectedly found that, thanks to the structural features of the belt structure, the tire of the invention achieves on the one hand curvature thrusts and drift adequate to the requirements, provided by the combination of carcass plies with a suitable crown angle with the radially inner belt layer incorporating inclined cords and the outer belt layer incorporating the zero-degree cords, and on the other hand a uniform and regular wear provided by the radially outer belt layer comprising zero-degree cords wound on the radially inner layer according to a preferred winding direction.
Advantageously, the cross-plies carcass lends the tire transverse strength and curve stability, while the aforesaid belt structure lends the tire dimensional stability, directional stability and lower energy absorption, achieving at the same time a low rolling resistance and a substantial disappearance of the chunking phenomenon.
In particular, it was observed that the presence in the belt structure of a radially outer layer provided with zero-degree cords allows to increase both the directional stability and the ground-contacting area of the tire, i.e. the area of contact with the ground, in all the operating conditions of the tire.
Thanks to this feature, both the stresses due to scraping on the ground and the stresses due to hysteresis dissipation in the rubber composition of the tread are reduced, with a substantial reduction of the tire overheating due to the centrifugal force which develops during the running, counter-balanced by the substantial inextensibility of the radially outer layer provided with zero-degree cords of the belt structure.
Furthermore, the adoption of such a combination of carcass and belt structures contributes to reduce the tire weight, with all of the ensuing advantages in terms of reduction of the inertia effects due to the nonsuspended masses.
According to a particularly preferred embodiment of the invention, the carcass structure of the tire comprises at least a couple of superposed plies, each provided with reinforcing cords parallel to one another and oriented according to a direction inclined and opposite, preferably symmetrically, with respect to the equatorial plane of the tire with respect to the cords of the adjacent ply.
Preferably, the reinforcing cords incorporated in the ply (plies) of the carcass structure form a crown anglexe2x80x94measured at the equatorial plane of the tirexe2x80x94not smaller than 25xc2x0 and, more preferably, comprised between 25xc2x0 and 70xc2x0 with respect to the equatorial plane of the tire.
In this way, an optimum curve behavior of the tire was observed, with the development of an adequate camber thrust.
In an alternative embodiment of the invention, it proves advantageous to insert between the carcass plies at least one sheet made of an elastomeric material as the crown angle formed by the reinforcing cords incorporated in the same plies reduces itself.
Such sheet may possibly incorporate adequate binding means suitable to increase its stretcheability properties without substantially altering the adhesion properties of the elastomeric material in the green state.
In this way, it is possible to absorb the tearing stresses which are developed between the plies of the carcass structure during rolling of the tire by means of a rather thin sheet having a thickness comprised between 0.075 mm and 0.5 mm.
Preferably, the aforesaid binding means comprises the so-called aramid pulp (short fibrillated fibers of poly-paraphenylene-terephtalamide), of the type commercially known as xe2x80x9cKevlar(copyright)-pulpxe2x80x9d or xe2x80x9cTwaron(copyright)-pulpxe2x80x9d (Kevlar and Twaron are registered trademarks of DuPont and Akzo, respectively).
Preferably, said short fibrillated fibers are incorporated in the elastomeric material that makes up said sheet, in an amount comprised between 1 and 10 parts by weight per each 100 parts by weight of rubber composition (phr), and have a length comprised between 0.1 mm and 2.5 mm.
According to the invention, the belt structure coaxially extending around the carcass structure comprises a radially inner layer including at least one strip made of an elastomeric material incorporating a plurality of reinforcing cords essentially parallel to one another and oriented along directions inclined with respect to the equatorial plane of the tire.
Preferably, the reinforcing cords incorporated in the strip of the radially inner layer of the belt structure are symmetrically inclined with respect to the cords of the radially outer ply of the carcass structure.
Preferably, furthermore, the reinforcing cords incorporated in the strip of the radially inner layer of the belt structure are essentially constituted by high modulus fibers of a material selected from the group comprising: nylon, aramid, polyethylene naphthalene 2,6 dicarboxylate (PEN), polyethylene terephthalate (PET), and glass.
Still more preferably, the aforesaid reinforcing cords are essentially constituted by poly-para-phenylene-terephtalamide, commercially known as Kevlar(copyright) (DuPont trademark).
Preferably, the reinforcing cords incorporated in the strip of the radially inner layer of the belt structure form a crown anglexe2x80x94measured at the tire equatorial planexe2x80x94not smaller than 25xc2x0, and still more preferably, comprised between 25xc2x0 and 75xc2x0 with respect to the equatorial plane of the tire.
In this way, an advantageous increase was observed in the effect carried out by the belt and/or carcass structures of containing the thrusts due to the centrifugal force.
Preferably, the reinforcing cords incorporated in the carcass ply (plies) or in the strip of the radially inner layer of the belt structure are made of a material different from the material of the reinforcing elements of the radially adjacent ply or strip.
In such a case, it is preferable to select materials having a modulus that increases by radially proceeding toward the outside; in so doing, a further advantageous increase was observed in the effect carried out by the belt structure of containing the thrusts due to the centrifugal force.
In an embodiment of the invention, the radially inner layer of the belt structure may further comprise an auxiliary supporting element essentially constituted by a sheet made of an elastomeric material incorporating appropriate binding means suitable to increase its stretcheability properties without substantially altering the adhesion properties of the elastomeric material in the green state.
In this way, it is advantageously possible to suitably space the zero-degree cords (preferably made of metal) from the reinforcing cords of the radially inner layer of the carcass structure, reducing possible fatigue phenomena on the carcass cords and interpenetration between the zero-degree cords and the reinforcing cords of the radially inner layer of the carcass structure.
Preferably, the aforesaid binding means comprises the so-called aramid pulp (short fibrillated fibers of poly-paraphenylene-terephtalamide), of the type commercially known as xe2x80x9cKevlar(copyright)-pulpxe2x80x9d or xe2x80x9cTwaron(copyright)-pulp (Kevlar and Twaron are registered trademarks of DuPont and Akzo, respectively).
Preferably, said short fibrillated fibers are incorporated in the elastomeric material that makes up said sheet, in an amount comprised between 1 phr and 10 phr, and have a length comprised between 0.1 mm and 2.5 mm.
Advantageously, said sheet made of an elastomeric material has a thickness comprised between 0.5 mm and 3 mm.
Preferably, said short fibrillated fibers are preoriented, for example by means of a calendering operation, according to the main direction of the forces which the supporting element is subjected to during the tire manufacturing method. Such direction is usually the circumferential direction of the tire and the aforementioned preorientation operation is preferably carried out by calendering said sheet during the manufacture thereof.
The elastomeric material reinforced with said aramid pulp has, at the green state, a tensile stress at break comprised between 3 MPa and 7 MPa, with a 50% elongation at a tensile stress comprised between 0.6 MPa and 3 MPa, while the same elastomeric material without aramid pulp has, at the green state, a tensile stress at break comprised between 1 MPa and 2 MPa, with a 50% elongation at a tensile stress comprised between 0.2 MPa and 0.5 MPa.
Preferably, the material that constitutes said auxiliary supporting element is a natural rubber-based composition containing carbon-black in an amount comprised between 30 phr and 70 phr, filled with the usual ingredients known in the art (plasticizers, protecting agents, antidegradation agents, vulcanizers), so as to obtain an elastomeric matrix as similar as possible to that of the elements to which said sheet has to adhere.
As said hereinabove, the radially outer layer of the belt structure comprises a plurality of circumferential coils, axially placed side by side, of at least one inextensible cord, circumferentially wound at a substantially null angle with respect to the equatorial plane of the tire, and usually known in the art by the term of xe2x80x9czero-degree cordsxe2x80x9d.
According to the invention, the winding direction of the cord(s) used to form the aforesaid coils is adapted to compression-stress the portions made of an elastomeric material interposed between consecutive reinforcing cords incorporated in the strip of the radially inner layer.
In other words, the winding direction of the cord is such that none of the reinforcing cords incorporated in the belt strip of the radially inner layer is so stressed as to move away from the adjacent cords, causing an undesired tearing stress that might separate from one another adjacent portions made of an elastomeric material.
Advantageously, this is obtainedxe2x80x94as will be better apparent later onxe2x80x94by winding during the manufacture the zero-degree cord in a direction opposite to the rotation direction of the assembly drum, and in such a way that the stress imparted during the winding to the cords incorporated in the strip of the underlying radially inner layer of the belt structure has a component along a direction perpendicular to the cords and oriented toward the equatorial plane of the tire.
Advantageously, furthermore, even without any auxiliary supporting element, the belt strip can effectively hold the zero-degree cord(s) during the winding.
Preferably, the zero-degree cord or cords of the radially outer layer of the belt structure are high-elongation metal cords made of high carbon content steel wires.
Alternatively, the zero-degree cords may be aramid textile cords.
When a tire to be mounted on the rear wheel of a motor-bike is to be manufactured, the cord coils arranged at a substantially null angle with respect to the equatorial plane of the tire are preferably distributed with a constant density throughout the axial development of the belt structure.
Instead, when a tire to be mounted on the front wheel of a motor-bike is to be manufactured, the cord coils arranged at a substantially null angle with respect to the equatorial plane of the tire are preferably distributed with a variable density throughout the axial development of the belt structure.
According to this last embodiment of the invention, the distribution density of the cord coils progressively changes along the layer, from the equatorial plane toward the ends, preferably according to a prefixed relation, having a value not higher than 15 cords/cm in an area placed on either side of the equatorial plane of the tire.
In this way, it is advantageously possible to obtain a belt structure which is at the same time flexible in the middle, to absorb and damp the vibrations due to the ground roughness, and rigid along the sides, to develop high slip thrusts.
According to the experiments of the Applicant, such relation may conveniently have the following expression:   Nx  =      K    ⁢                  R        2                    r        2              ⁢    No  
wherein:
No is the number of cord coils arranged in a central portion of unitary length located on either side of the equatorial plane of the belt structure;
R is the distance between the center of said portion and the rotation axis of the tire;
r is the distance between the center of the generic unitary portion comprised between the equatorial plane and the ends of said radially outer layer of the belt structure and the rotation axis of the tire;
K is a parameter that takes into account the constituent material and the formation of cords, as well as the amount of rubber around the cord, and the weight of a portion of the radially inner layer of the belt structure at said unitary portion, which is variable with variations in the material type and in the structural features of the radially inner layer of the belt structure along the crown profile that diverge from the reference values.
This parameter K may have a value of substantially close to 1 if all the cords have the same formation and all the connected materials are the same throughout the layers, or different values according to the variations in the materials and formation of the reinforcing elements along the peripheral development of the belt structure.
A distribution of the cords in accordance with such relation ensures both uniformity of the stress acting on the belt structure during use of the tire as a consequence of the centrifugal force applied, and the necessary differentiated stiffness along the axial direction.
Obviously, those skilled in the art may find other relations which, depending on the aforesaid design variables, would allow to obtain at the same time a differentiated stiffness along the axial direction and a stress uniformity in the belt structure of the running tire, by varying in a controlled and predetermined manner the density of the aforesaid cords.
As to the density of the zero-degree cords in the portion located on either side of the equatorial plane, where the maximum thinning out takes place, this density, for a tire to be mounted on the front wheel, is preferably not greater than 8 cords/cm and more preferably comprised between 3 cords/cm and 6 cords/cm.
For a tire to be mounted on the front wheel, the width of said portion varies preferably from 10% to 30% of the axial development of the belt.
Conveniently, for a tire to be mounted on the front wheel, the quantity of cords in said central portion is equal to a value comprised between 60% and 80% of the cords quantity near the shoulders of the tire, where the density of said cords is preferably not greater than 10 cords/cm and more preferably included between 6 cords/cm and 8 cords/cm.
According to a second aspect, the invention also provides a method for the manufacture of a high transverse curvature tire as described above, comprising the steps of:
providing on a main assembly drum a carcass structure comprising at least one ply provided with reinforcing cords essentially parallel to one another and oriented along directions inclined with respect to the equatorial plane of the drum;
providing on a comb-like drum at least one strip made of an elastomeric material incorporating a plurality of reinforcing cords essentially parallel to one another and oriented along directions inclined with respect to the equatorial plane of the drum;
imparting to said at least one strip a curvilinear transverse profile by radially expanding a plurality of radially mobile sectors associated to the comb-like drum;
circumferentially winding on said at least one strip at least one inextensible cord, so as to form a plurality of circumferential coils consecutively arranged side by side, and to define a belt structure having a curvilinear transverse profile;
circumferentially associating a tread around the belt structure;
associating the belt structure, together with said tread, around the carcass structure,
wherein said at least one inextensible cord is wound on said at least one strip of the belt structure at a substantially null angle with respect to the equatorial plane of the comb-like drum and according to a winding direction adapted to compression-stress portions made of an elastomeric material interposed between consecutive reinforcing cords incorporated in said at least one strip of the radially inner layer.
In a preferred embodiment of the method, the inextensible cord is wound on the strip of the belt structure in such a way as to obtain cord coils distributed with variable density, preferably in accordance with the above explained distribution law, throughout the axial development of the belt structure.
Advantageously, as explained above, it was found that the belt strip can effectively retain the zero-degree cord or cords during the winding step, even without auxiliary elements designed to increase the adhesion properties of the zero-degree cords on the same.
According to an alternative embodiment, the method of the invention may further include the step of applying on the strip of the belt structure an auxiliary supporting element essentially constituted by a sheet made of an elastomeric material, in particular and preferably incorporating appropriate binding means suitable to increase its features of stretcheability without substantially altering the adhesion properties thereof.
According to still a further embodiment, the method of the invention may comprise the additional step of applying a sheet made of an elastomeric material, between the carcass plies, with the purposexe2x80x94as explained hereinabovexe2x80x94of absorbing the tearing stresses that are developed between the aforesaid plies during tire rolling.
Advantageously, said sheets made of an elastomeric material incorporate binding means having the features described hereinabove.